1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device for providing a display by reflecting incident light and a method for producing the device.
2. Description of the Related Art
In recent years, applications of liquid crystal displays in word processors, laptop personal computers, pocket televisions and the like have advanced. Among liquid crystal display devices, a reflection type liquid crystal display, which provides a display by reflecting incident light, and thus requires no back light source, has received considerable attention because it can realize low power consumption and can be made thin and miniaturized.
Conventionally, reflection type liquid crystal display devices have employed a twisted nematic (TN) mode or super-twisted nematic (STN) mode as a display mode. However, these display modes require a polarizer. Consequently, half of the incident light is not utilized for the display, resulting in a darker display.
To obtain a brighter display, display modes without polarizers have been proposed in which all of natural incident light is effectively utilized. As an example, a phase transition type guest-host mode is utilized whereby a cholesteric-nematic phase transition phenomena is caused by an electric field (see, e.g., D. L. White and G. N. Taylor, J. Appl. Phys. 45, pp. 4718-4723, 1974). Also, a multi-color reflection type display in which a micro color filter is used in combination with the guest-host mode has been proposed (see, e.g., Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29. pp. 157-160, 1988).
To realize an even brighter display in a display mode which does not require a polarizer, it is necessary to increase the intensity of scattered light in a direction perpendicular to a display screen for the incident light at every incident angle. This requires a reflector which has optimum reflective characteristics. The above-mentioned article of White et. al. describes a reflector which is manufactured by roughening a surface of a substrate such as glass by using abrasive; controlling the surface asperities by varying the time of etching with hydrochloric acid; and forming a silver foil on the asperities of the surface.
In a case where a conventional active matrix substrate 20, as shown in FIG. 1, is used for a reflection type liquid crystal display, the active matrix substrate 20 on which driving devices are formed is required to function as a reflector. The structure of the conventional active matrix substrate 20 will be described below.
FIG. 1 shows in relevant portion the active matrix substrate 20 which includes a thin film transistor (TFT) 1 as a switching element. FIG. 2 is a cross-sectional view taken along the line V--V in FIG. 1. As shown in FIGS. 1 and 2, the active matrix substrate 20 includes an insulator substrate 2 such as glass, for example, and a plurality of gate bus lines 3 made of chromium, tantalum or the like, which are disposed in parallel on the insulator substrate 2. Gate electrodes 4 branch off from each of the gate bus lines 3. The gate bus lines 3 function as scanning lines as is conventional.
A gate insulating film 5 is formed on the entire surface of the insulator substrate 2 covering the gate bus lines 3 and the gate electrodes 4. The gate insulating film 5 is made of silicon nitride (SiNx), silicon oxide (SiOx), or the like. A semiconductor layer 6 made of amorphous silicon (a-Si), polycrystalline silicon (p-Si), cadmium selenide (CdSe), or the like is formed on the gate insulating film 5 in each area thereof above the gate electrode 4. A source electrode 7 made of titanium, molybdenum, aluminum, or the like is disposed so as to overlap one end of the semiconductor layer 6 in each area. Similarly, a drain electrode 8 made of titanium, molybdenum, aluminum, or the like is disposed to overlap the other end of the semiconductor layer 6 in each area. A pixel electrode 9 is formed to overlap an opposite end of the drain electrode 8 against the end on which the semiconductor layer 6 is overlapped. The pixel electrodes 9 are a transparent conductive film made of a material such as indium tin oxide (ITO).
As shown in FIG. 1, a plurality of source bus lines 10 are disposed to cross the gate bus lines 3 with the gate insulating film 5 interposed therebetween. The source electrodes 7 are connected to respective source bus lines 10. The source bus lines 10 are made of the same material as that of the source electrodes 7 and function as signal lines. The TFT 1 is formed by the gate electrode 4, the gate insulating film 5, the semiconductor layer 6, the source electrode 7 and the drain electrode 8. The TFT 1 functions as a switching element as will be appreciated.
In order to use the active matrix substrate 20 for a reflection type liquid crystal display, not only the pixel electrodes 9 are required to be made of metal exhibiting a reflection property such as aluminum or silver, but also the gate insulating film 5 is required to have a scattering property, for example, to have asperities on its surface. Nevertheless, it is difficult to form asperities uniformly on the surface of the gate insulating film 5 which is made of an inorganic material.
Regarding a method for increasing the intensity of light scattered in the direction perpendicular to the display screen, it has been proposed in UK Patent Application No. 2 066 545 A to use a metallic thin film having surface asperities as a reflector. FIGS. 3A to 3D and 4A to 4F show variations of forms of the surface asperities (roughened surface) of the metallic thin film described in the article. The degree of whiteness of the appearance of the roughened surface varies as the height (H) and period (L) of the asperities vary (as discussed in more detail with reference to FIGS. 5A to 5C).
Accordingly, these factors H and L should be controlled well in order to realize an optimum reflection property.
As shown in FIG. 5A, in the case where L&gt;&gt;H, reflection components of the light from the metallic thin film are greater than scattering components. Thus the metallic thin film has a specular surface. In the case where L=H, as shown in FIG. 5B, the scattering components are dominant and the metallic thin film exhibits a white surface. In the case where L&lt;&lt;H, as shown in FIG. 5C, the incident light is absorbed by troughs formed by the asperities of the metallic thin film. Thus the metallic thin film exhibits a gray to black surface. In FIGS. 5A to 5C, the height H is approximately 0.01 .mu.m to 2.0 .mu.m.
In the case where the metallic thin film 51 has a height H greater than a period L, a reflector 50 may have a laminated structure as shown in FIG. 6. The reflector 50 includes a thin insulating layer 53 and another metallic thin film 52 formed on the metallic thin film 51, resulting in white reflected light from the reflector 50 including laminated thin film layers. The insulating layer 53 is made of a material such as SiO.sub.2 or Si.sub.3 N.sub.4 and is formed on the metallic thin film 51 by using a chemical vapor deposition (CVD) technique or sputtering technique. The metallic thin film 52 having a roughened surface is formed on the insulating layer 53.
A metallic thin film may be formed by using one of the following techniques:
(1) Vacuum evaporation or sputtering technique. PA1 (2) Vacuum evaporation or sputtering technique with subsequent heat treatment and recrystallization. PA1 (3) Vacuum evaporation or stuttering technique for forming an alloy thin film, then heat treatment for precipitation (separation), and etching for removing precipitated particles from the surface of the alloy thin film.
The first technique makes the asperities of the metallic thin film surface by controlling conditions of the vacuum evaporation or sputtering when the metallic thin film is formed on a substrate. The temperature of the substrate is kept high (more than 100.degree. C.) and a very small amount of water is to be contained in the atmosphere in which the metallic thin film is deposited.
The second technique makes the asperities of the metallic thin film surface by heating and recrystallizing the metallic thin film which is formed on the substrate by vacuum evaporation or sputtering. In the event aluminum or aluminum alloy is used for the metallic thin film, recrystallization occurs at a temperature ranging from 100.degree. C. to 600.degree. C., as the melting point is 660.degree. C. The recrystallization causes atoms included in the metallic thin film to be rearranged, resulting in a roughened surface thereof.
In the third technique, as shown in FIG. 7, an alloy thin film 63 is formed on a substrate 61 using vacuum evaporation or a sputtering technique. Then, the alloy thin film 63 is heated to precipitate particles 64 from the alloy thin film 63. The surface of the alloy thin film 63 is removed by etching, resulting in asperities of the surface.
In the case where the metallic thin film 63 is an aluminum alloy which contains 2% by weight of silicon and is heated for 20 minutes in a N.sub.2 atmosphere at 400.degree. C., precipitated particles 64 of an intermetallic compound of aluminum and silicon have diameters ranging from 0.2 .mu.m to 1.0 .mu.m. For example, by heating and precipitating the metallic thin film 63 having a thickness of 1.0 .mu.m and by etching off the surface thereof by 0.2 .mu.m, the metallic thin film 63 exhibits white color (i.e., the light reflected by the surface of the metallic thin film 63 appears white).
In addition, the U.K. article suggests that the surface of a metallic thin film can be roughened by sand blasting or shot blasting.
As shown in FIG. 2, the active matrix substrate 20 has concave and convex portions and comparatively large steps on the surface due to driving devices such as switching elements, capacitors, and the like. These concave and convex portions and large steps undesirably affect an alignment film (not shown) which is formed on the surface of the substrate 20. In order to avoid this problem and to improve the effect of the alignment film, the article suggests that a transparent film may be provided on the substrate 20 covering driving devices formed on the substrate 20, so as to make the surface of the substrate 20 level. The transparent film may be made of organic resin such as silicon resin, epoxy resin and polyamide resin, or inorganic resin.
The U.K. article also suggests that a white insulating layer 80 (shown in FIG. 8) may be formed in place of, or in addition to, the metallic thin film in order to make the substrate have a white appearance. The white insulating layer 80 is made of transparent organic resin 82 including fine metal oxide particles 81.
In order to obtain a white appearance, the U.K. article suggests another method in which the surface of the metallic thin film made of aluminum or aluminum alloy is anodized to form an alumina (Al.sub.2 O.sub.3) layer. For example, as shown in FIG. 9, by anodizing an aluminum or aluminum alloy layer 92 formed on a substrate 91, an alumina layer 93 having a thickness of 5 .mu.m to 30 .mu.m is formed. The alumina layer 93 has a honeycomb structure. Incident light 94 is scattered by interfaces 96 of the honeycomb structure as well as the surface of the alumina layer 93 as shown in FIG. 9, resulting in a white appearance. The alumina layer 93 insulates an electric current, so that it improves reliability of a liquid crystal display in which driving electrodes are directly in contact with the liquid crystal.
However, it is difficult to form uniform asperities of the surface of the metallic thin film by using the above mentioned techniques described in the aforementioned articles, because precipitation and etching processes have an unpredictable nature.
The white appearance of reflected light from a reflector (such as a face of substrate or a metallic thin film) means that the incident light is reflected and scattered in all directions and at all angles. In a case where the reflector includes electrodes for driving the liquid crystal and is formed on an active matrix substrate in contact with the liquid crystal, the reflected light goes through the liquid crystal and a counter substrate into the air. When the air has a refractive index of 1 and the substrates and the liquid crystal layer have a refractive index of 1.5, reflected (scattered) light from the reflector which enters an interface of the substrate and the air with an incident angle greater than about 48.degree. is reflected by the interface so that the light does not travel out from the display. Accordingly, a "white" reflector which reflects and scatters the incident light in all directions and at all angles makes the display darker by not utilizing some components of reflected light.
A reflector for realizing a brighter display needs to have directional properties for controlling the reflected light. Nevertheless, it is difficult to form a reflector having the directional properties by using the above mentioned techniques because of the unpredictable nature of the processes.
Reflective characteristics of a reflector formed by heating an aluminum or aluminum alloy layer in an inert gas at a temperature of 400.degree. C. to 450.degree. C. are relatively specular so that a liquid crystal panel utilizing the reflector appears dark (see, for example, Japanese Laid Open Patent No. 56-156864). In order to obtain more scattering properties of the reflector (i.e., highly rugged surface), the aluminum or aluminum alloy layer needs to be heated at a higher temperature.
However, switching elements used for driving a liquid crystal layer cannot stand such a high temperature. For example, a-Si TFTs formed on a glass substrate are destroyed at a temperature higher than 350.degree. C. where hydrogen contained in the semiconductor layer of the TFT leaves therefrom. In a case of MIM (metal-insulator-metal) elements which are formed by anodizing metal such as tantalum, the MIM elements are also destroyed at such a high temperature where incomplete oxidized tantalum ions in an interface between a tantalum layer and an anodized tantalum layer diffuse into the anodized layer.
Accordingly, with respect to a reflector used for a liquid crystal display comprising such switching elements as mentioned above, heating processes are required to be performed at a temperature lower than 300.degree. C.
In addition, a reflector formed by using the above mentioned precipitation technique appears dark when a liquid crystal layer is mounted thereon (see Japanese Laid Open Patent No. 56-156864).
The above mentioned Japanese Laid Open Patent also describes a method for forming a reflector which has directional properties for controlling the reflected light, in the case where the reflector is used in an active matrix substrate including switching elements such as a-Si TFT or MIM elements. According to this method, a SiO.sub.2 layer is formed on a substrate as shown in FIG. 10 so as to have a sinusoidal wave shape in its cross section by using CVD technique, and an aluminum layer is formed to coat the surface of the SiO.sub.2 layer. The reflector has an average angle .theta. of 5.degree. to 30.degree..
U.S. Pat. No. 4,431,272 also describes a reflector (aluminum electrode) in which an aluminum layer is formed on a rugged SiO.sub.2 layer. The reflector has asperities on the surface thereof as shown in FIG. 11.
U.S. Pat. No. 5,408,345 describes a reflector formed by a metallic thin film represented by a plurality of reflection electrodes formed on an organic insulating film which is formed on a substrate covering switching elements and bus lines. The organic insulating film has asperities in a surface region thereof. The organic insulating film is formed by coating photo sensitive resin on a substrate; exposing the photo sensitive resin layer to light by using a photo mask which has circular light shielding portions; and developing the photo sensitive resin layer followed by a heating process, so as to form a plurality of bumps.
As described above, reflectors having asperities are obtained by forming a lower layer having a rugged surface (bumps, or small concave and convex portions), followed by forming a metallic (specular) thin film on the rugged lower layer, rather than by heating the metallic thin film to obtain scattering properties.
A conventional method for fabricating a phase transition type guest-host liquid crystal display using a vertical alignment film is as follows:
Reflective pixel electrodes of aluminum are formed on a substrate on which driving devices such as switching elements, bus lines and the like has been already formed. A vertical alignment film is formed by printing or coating an alignment film material on the substrate covering at least the reflective pixel electrodes, followed by a heating process at a predetermined temperature so as to make an active matrix substrate. Then an adhesive sealing material including, for example, spacers of 7 .mu.m diameter is applied on the active matrix substrate by using screen printing. The active matrix substrate is then combined and adhered with a counter substrate which has counter electrodes of ITO and a vertical alignment film formed thereon. Liquid crystal is filled in a space formed between the active matrix substrate and the counter substrate.
However, in a liquid crystal display fabricated according to the above mentioned method, the liquid crystal between the substrates has a focal conic texture as shown in FIG. 12. The liquid crystal having such as a focal conic texture requires a relatively higher driving voltage and the response speed thereof is low.